Hydraulic fracturing is a standard practice in stimulating production of hydrocarbon products from hydrocarbon reservoirs. During hydraulic fracturing treatment, pressurized fluids are injected into a wellbore to overcome a breaking strength of rock. Consequently, one or more hydraulic fractures are initiated that subsequently propagate away from the wellbore into the reservoir until fluids injection stops. Eventually, the created hydraulic fractures serve as conductive pathways through which hydrocarbon products migrate en route to the wellbore and are brought up to the surface. Hydraulic fracturing has also been applied for preventing sand production in unconsolidated rock formations and for stimulating heat extraction from geothermal system.
Hydraulic fracturing is commonly executed in cased wellbores. A cased wellbore is constructed first by drilling a borehole (known as an open-hole wellbore) to a target depth. Then, steel casing is arranged and placed in the borehole and an outside of the casing is bonded with the borehole using cement. After the casing is cemented (i.e., open-hole wellbore is made into cased wellbore), downhole tools (e.g., perforation guns) are used to create holes at a wall of the wellbore that penetrates the casing, cement sheath, and some distance into the rock formation associated with a reservoir (or reservoir rock). The purpose of perforating the cased wellbore is to establish hydraulic communication between reservoir rock and the cased wellbore. Finally, the cased wellbore or a portion thereof is hydraulically fractured by injecting pressurized fluid.
Pre-existing weak planes are common features of geological formations, exhibited in a variety of forms. The weak plane is formed through brittle failure of rock mass as a result of tectonic movement. It is well-understood that the rocks containing interlocked or weakly bonded planes have weaker strengths than rock formations lacking such an interface. Correspondingly, the plane serves as a preferential plane of failure. As such, the interlocked or weakly bonded plane is often called weak plane. Besides, the failure of the weak plane is governed by frictional contact of the surfaces, and slip (or shear movement parallel to the surfaces) occurs upon reaching a critical condition. Typical forms of weak planes are joints and faults. Despite having a similar origin as faults, the joints are also called natural fractures. However, both forms may consist of multiple fractures with specific dominating strike and dip orientations. For clarity purposes, “fault” is used herein as a generalized term representing all types of pre-existing weak planes in the subsurface.
The slip of the fault (also referred as fault reactivation) can be detrimental to wellbore integrity under some extreme circumstances. In hydraulic fracturing or water injection operations, seismic monitoring is performed to capture the seismic event generated during shear slip of weak planes. If the observed seismic events occur at small magnitudes (i.e., classified as micro-seismic event) and far away from wellbores, the wellbore system composed of casing and cement sheath is capable to sustain such mechanical loading induced by the slip of weak planes. However, field observations have shown that in some tectonically active regions, if the wellbore is drilled through a fault and the induced slip is greater than a certain magnitude (such as, a few centimeters), the slip can lead to severe deformation of a steel casing that is cemented inside a wellbore. This leads to the distortion of casing geometry, resulting obstruction inside casing along the wellbore pathway that makes subsequent downhole operations impossible. This further brings environmental and safety concerns over the risk of wellbore integrity during the lifetime of the wellbore. Currently, no viable approaches are available that can effectively reduce the risk of fault reactivation induced casing damage during hydraulic fracturing once the casing is cemented inside a wellbore.
Based on above description, methods and tools for mitigating fault reactivation induced casing damage for cased wellbore are desired, especially methods and tools that are compatible with current field practices and procedures.